1. Field of the Invention
The invention relates to a method of manufacturing a unit fuel cell.
2. Description of Related Art
There is known a method of manufacturing a unit fuel cell which includes a membrane electrode assembly in which electrode catalyst layers are formed at opposite sides of an electrolyte membrane, a first gas diffusion layer disposed on one side surface of the membrane electrode assembly, a second gas diffusion layer disposed on the other side surface of the membrane electrode assembly and a support frame configured to support the membrane electrode assembly in an outer periphery of the membrane electrode assembly, the method including: a process of forming a thermoplastic adhesive layer on an outer peripheral edge portion of the other side surface of the membrane electrode assembly; a process of disposing the first gas diffusion layer on one side surface of the membrane electrode assembly, disposing the second gas diffusion layer on the other side surface of the membrane electrode assembly so that an outer peripheral edge portion of the second gas diffusion layer overlaps with an inner portion of the adhesive layer; a process of heating, compressing and integrating the first gas diffusion layer, the membrane electrode assembly, the inner portion of the adhesive layer and the second gas diffusion layer; a process of disposing an inner peripheral edge portion of the support frame on an outer portion of the adhesive layer; and a process of heating, compressing and integrating the support frame, the outer portion of the adhesive layer and the membrane electrode assembly (see, e.g., Japanese Patent Application Publication No. 2014-029834 (JP 2014-029834 A)).
In JP 2014-029834 A, the outer peripheral edge portion of the other surface of the membrane electrode assembly is covered by the adhesive layer. Thus, the outer peripheral edge portion of the membrane electrode assembly is not exposed to a supplied gas from a gap between the support frame and the second gas diffusion layer. This makes it possible to avoid generation of cracks in the membrane electrode assembly which may otherwise occur when exposed. However, there is a possibility that the support frame made of a resin is thermally deformed when heating, compressing and integrating the support frame, the outer portion of the adhesive layer and the membrane electrode assembly. Accordingly, in order to deal with this possibility, the present inventors have tried to use an adhesive agent cured by ultraviolet rays, namely an ultraviolet-curable adhesive agent. Since ultraviolet rays cannot pass through a gas diffusion layer, it is necessary to cure the adhesive agent prior to disposing the gas diffusion layer in a membrane electrode assembly. That is to say, it is necessary to form an adhesive layer by coating an ultraviolet-curable adhesive agent on an outer peripheral edge portion of a membrane electrode assembly, dispose an inner peripheral edge portion of a support frame on an outer portion of the adhesive layer, and then cure the adhesive layer with ultraviolet rays prior to disposing an outer peripheral edge portion of a second gas diffusion layer on an inner portion of the adhesive layer. However, in this case, if one tries to dispose the second gas diffusion layer on the adhesive layer cured by the ultraviolet rays, the outer peripheral edge portion of the second gas diffusion layer is propped up by and overlapped with the upper portion of the inner portion of the adhesive layer. Thus, the outer peripheral edge portion of the second gas diffusion layer is raised more upward than the remaining portion of the second gas diffusion layer. As a result, there is generated a phenomenon that flow paths of a supplied gas defined by the second gas diffusion layer and a separator existing thereon are closed in the raised portion, fibers making up the second gas diffusion layer are made fluffy in the raised portion, or an excessive surface pressure is applied to the membrane electrode assembly existing under the adhesive layer. Thus, there is a possibility that the battery performance is reduced. On the other hand, if the amount of the adhesive agent coated on the outer peripheral edge portion of the membrane electrode assembly is made small so that the adhesive layer should not reach the underside of the gas diffusion layer, there is a possibility that the outer peripheral edge portion of the membrane electrode assembly is partially exposed. A demand has existed for a technique capable of reliably integrating the support frame, the gas diffusion layer and the membrane electrode assembly without exposing the membrane electrode assembly and without deforming the support frame.